<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<style>
    html{
        font-size: 100px;
    }
    div{
        background: #ccc;
        margin-top: 10px;
        font-size: .16rem;
    }
</style>
<body>
<p style="font-size: 0.1rem">rem1</p>
<p style="font-size: 0.2rem">rem1</p>
<p style="font-size: 0.3rem">rem1</p>
<div style="width: 1rem">div1</div>
<div style="width: 2rem">div2</div>
<div style="width: 3rem">div3</div>
</body>
</html>